In conventional CDMA systems, intra-cell interference contributes to the major portion of the total interference in the Reverse Link (RL). Tight (fast) RL power control with dedicated power-control sub-channels are required to ensure a proper signal-to-noise (SNR) level of a mobile terminal and to minimize its interference to other users due to the well-known near-far problem.
In currently existing 3G CDMA systems, in order to minimize intra-cell interference, the fundamental power control (PC) mechanism involves two control loops, an inner loop and an outer loop, which have been inherited from 2G systems for circuit voice applications. FIG. 1 shows these conventional prior art power control loops that exist between a mobile Access Terminal (AT) 101, a Base Transceiver Station (BTS) 102, and a Radio Network Controller (RNC) 103. Fast power control is conducted by the inner loop that includes, within AT 101, a pilot power adjuster 104, and within BTS 102, a pilot measurer 105, which measures the power of the pilot received on the reverse link (RL) from AT 101, a pilot versus set point comparator 106, which generates power control bits (PCBs) based on comparisons of the measured pilot power against a target (set point), and a PCB transmitter 107, which transmits the PCBs generated by comparator 106 over the forward link (FL) to the pilot power adjuster 104 in AT 101. The power of the pilot transmitted by AT 101 is dynamically adjusted in response to the PCBs received from BTS 102, as is the power of the traffic channel (circuit voice), which is adjusted in response to the traffic power adjuster 108. The latter, dynamically adjusts its traffic output power according to the pilot power so as to maintain a fixed Traffic-to-Pilot Ratio (TPR), the latter being a fixed parameter that is inputted to adjuster 108. Since AT 101, which transmits circuit voice, does not receive quality feedback from BTS 102 (e.g., an ACK/NAK) (as in the case in CDMA2000 1x and EVDV), the power control set point used by comparator 106 is dynamically adjusted by the outer loop. The outer loop includes traffic power adjuster 108 within AT 101, traffic demodulator 109 and decoder 110 within BTS 102, and set point adjuster 111 within RNC 103. Specifically, the voice traffic received by BTS 102 is demodulated by demodulator 109 and is decoded by decoder 110. A cyclic redundancy check (CRC) is calculated by decoder 110 for each received and decoded digital voice frame, where the calculated CRC indicates whether a received frame has been decoded properly and “passes”, or is in error and “fails”. The pass/fail indications of the successive CRC calculations are outputted by decoder 110 and inputted to set point adjuster 111. Based on these pass/fail indications and a targeted frame error rate (FER), set point adjuster 111 dynamically adjusts the set point with which comparator 106 compares the measured received pilot. The AT pilot transmission power is thus dynamically controlled by the generated power control bits such that the received pilot at BTS 102 tracks the pilot set point.
For 3G CDMA systems supporting burst data services, some power control enhancements are disclosed in various co-pending patent applications. For example: in Ser. No. 10/924,268, filed Aug. 23, 2004, ACKs/NAKs are used to adjust the TPR of the burst data traffic without increasing any overhead; in Ser. No. 10/798,166, filed Mar. 11, 2004, a pilot frame error rate is used in the outer loop to determine the PC target of the fast inner loop when no voice traffic is available; and in Ser. No. 10/954,755 filed Sep. 30, 2004, a Channel Quality Indicator (CQI) erasure metric is used in the outer loop to determine the PC target of the fast inner loop when voice traffic is unavailable.
The above-described prior art mechanism involving inner and outer control loops is complicated and not really effective and efficient for systems with multi-flow applications including conversational streaming and burst types of traffic.
Unlike conventional CDMA systems where intra-cell interference contributes to the major portion of the total interference in the RL, in the new air interface proposed to CDMA2000 Revision C (RevC) standard, such as OFDMA (Orthogonal Frequency Division Multiple Access) and CDMA with Interference Cancellation (IC), intra-cell interference is not the major factor of concern for power control. In the RL of an OFDMA system, intra-cell interference is minimal while in the RL of a CDMA system with IC, intra-cell interference is progressively cancelled and, in fact, the receiving power difference among different ATs may aid the IC process. As tight a power control as is required in existing CDMA systems is thus not needed in the newer CDMA and OFDMA RevC systems. On the other hand, however, in a CDMA system with IC, the ATs whose RL signals are decoded earlier will still be impacted more by the intra-cell interference. Further, there is still inter-cell interference for both CDMA with IC and OFDMA systems. Thus, in order to maintain a sufficient signal-to-noise ratio (SNR), power control is still needed. In addition, in a new RevC system, inter-cell interference is also an important factor that needs to be considered for power control. An AT at a cell edge will need to be restricted on its transmission power based on its neighbor cell's tolerance on inter-cell interference.
A simpler power control mechanism is thus needed for new RevC CDMA and OFDMA systems where as tight control as is presently being used in conventional systems is not required.